The goal of this similarity is to help users who would like to keep the
functionality of the `tf-idf` similarity that we want to remove, or to allow
for specific usec-cases (disabling idf, disabling tf, disabling length norm,
etc.) to not have to build a custom plugin and familiarize with the low-level
Lucene API.
We want to upgrade to Lucene 7 ahead of time in order to be able to check whether it causes any trouble to Elasticsearch before Lucene 7.0 gets released. From a user perspective, the main benefit of this upgrade is the enhanced support for sparse fields, whose resource consumption is now function of the number of docs that have a value rather than the total number of docs in the index.
Some notes about the change:
- it includes the deprecation of the `disable_coord` parameter of the `bool` and `common_terms` queries: Lucene has removed support for coord factors
- it includes the deprecation of the `index.similarity.base` expert setting, since it was only useful to configure coords and query norms, which have both been removed
- two tests have been marked with `@AwaitsFix` because of #23966, which we intend to address after the merge
* Add docs with up to date instructions on updating default similarity
The default similarity can no longer be set in the configuration file
(you will get an error on startup). Update the docs with the method
that works.
* Add instructions for changing similarity on index creation
The default similarity was set to `classic` which refers to TFIDF and has not been moved after the upgrade to Lucene 6.
Though moving to BM25 could have some downside for queries that relies on coordination factor (match_query, multi_match_query) ?
relates #18944
Adrien Grand
Adriel Dean-Hall
Jim Ferenczi
eratio08
Robert Muir
Clinton Gormley
William
Kevin Wang
Oleg Anashkin
Lee Hinman